check the sockets on the tms9928's. Pull the plastic piece off and give them a good cleaning - and make sure the 9928 fits snugly. Might do the same on the 6821 and main cpu as well. Sounds like somethings not telling the 9928's to draw the canoe and stuff.
I had a similar issue on mine (I recall it being the gators, granny and the natives). Verify the brightness is turned up on the monitor. I felt real smart after 2 hours of testing to find that out!
now that you mention that- i think there's a kind of balance pot, between the 2 9928's - that may need adjusting as well. Mark the position of the pots first, as that thing's a real bear to get adjusted right.